C&N&DIA.N AOURNAL OF EXPLORAITION GEOPHYSICS ““L, 30. NO, 2 ,DECEMBER 19941, P, 103~114 PETROLEUM GEOLOGY OF NEPAL’ OR. FRIEDENREICH*, O.L. SLIN$, U.M.S. PRADHAN~ AND R.B. SHRESTHA~ ARSTRACT 3.0 PROCEDURF. The project was a team effort consisting of Alconsult Magnetic.gravity andseisn1ic surveys, exIcnSi”e geological field mapping programs. StrUCtUlaland straiigraphic studies and professionals in the fields of geology, geophysics (seismic, prtr”le”m grochrmicill in”esligati”nshave her” carried““I in ,hr gravity), sedimentology, petrography and photogeology Trrai plains and the adjacent Siwalik Fold Belt of southernNepal along with petroleum geochemists from the Geological sincethe lxc 1970s.This work. which has hem recentlyincorp ratedinto a comprehensiveevaluation. suggesrs that the countrylhas Survey of Canada and geologists from the Petroleum mrac,iYe petroleumpor,ihihties. Rocks with iignificam xn”rCe Exploration Promotion Project of Nepal. The work done by potential havebeen samplcd from oumops.and lhcmal modelling the team consisted of: a) reviewing all petroleum-related ruggcstshat, ill theprDspecli”r pm 01Ncpd ,hescpotential S,~“ICC rocksare in the oil andgas gmeration window. Effective resewoirr information on Nepal and environs: b) collecting and analyr- Emcxpeccml to beloud in IhCt,.owcr Siwalik. S”rkhCI amI Condrana ing oil and gas seep samples; c) conducting field traverses sandrtoncsand in the carhonatci of the I.akharpata~Vindhyan throughout southern Nepal to collect samples for geochemi- G*O”p. cal and petrographic analyses; d) field mapping (structure) in Enpl”rati”n “pp”nunities in the Trrai include Stl”Cl”lrSilSS”Ci~ selected critical areas; e) studying the sedimentology 01 atedwith blind thrusts.hascment~con,rollcd drape and bull ~b’uc- potential reservoir rocks; and f) reviewing and interpreting twes, stratigraphic pinch-outs and suhcroptrapr. Major folds and thrustsin the Siwalik Fold Belt arelikely to providestr~ctwal traps. all the existing Nepal geophysical data. Ddling dcplh* to ohjeclivcs arc hm 150010 5000 mctrc* in the Teraiand from 25011fo 4000rncms in the Siwalik Fold Bell. 4.0 DATA BASE The data base included all previous work in which the PEPP/DMG was involved. This data included: a) more than 1.0 INTRonUCTloN IO years of geological field mapping (Schroeder, 19X I ; This paper is based on the recently completed Nepal Source Shrestha and Shrestha. 1983); b) a regional aeromagnetic and Seal Study conducted by Alconsult International Ltd. survey (CCC. 1980); c) a photogeological study (Hunting (AIL) and the Nepal Petroleum Exploration Promotion Project/ Geology and Geophysics Ltd., 1984); d) results of several Department of Mines and Geology (PEPP/DMG) HMG of geochemical investigations (Kay&ha, 1989): e) information Nepal (Slind, 1993). Geochemical sample collecting, labora- from the Shell Biratnagar I well; and f) basic field data and tory analysis, interpretation and modelling was conducted by interpretations from four geophysical surveys: 1983. I984 the Geochemistry Section of the Institute of Sedimentary and World BankKGG 1250 km reflection seismic program Petroleum Geology (ISPG) of the Geological Survey of (Harris, 19X4), 1987.1988 PClAClAirborne 1651 km retlec- Canada. The study operated under the overall direction of tion seismic and gravity program (Friedenreich and Slind. Petro-Canada International Management Services (PCTMS) for 1989), 1988-1989 Shell Nepal 1940 km (615 km used in this the Canadian International Development Agency. interpretation) reflection seismic program, 194 l-1993 French Government/CGG 422 km retlection seismic program. 2.0 OtSJECTtVE STUDY 5.0 GEoCnAPHICAL/GEoLoCICAI. SETTING The main objective of the study was to better understand the petroleum geology of Nepal and to define its oil and gas The Kingdom of Nepal, an independent country lying potential by combining existing data with new information between India to the south and China to the north (Figure I), acquired during the course of the study. is 800 km long from east to west and ranges in width from ‘Presented at the CSEGKSPG Joint Convention, Calgary. Alhem, May I I. 1994. Manuscript received hy the Editor October 17. 1W4; wised manuscripr received January 23, 19% 2AI~~n~~It lntrmationai Ltd.. 450.5X1 6th Avenue SW.. Calgary. Alhem T2P US2 ?HMG Nepal Petroleum Exploration Promorion Project. Dcpartmcnt of Mines and Geology. Kathmandu. Nrpai Thanks are given fo Psrro-Canada lnterna&mal Management Services. prqject managers for Canadian International Developmcnl Agmcy. and IO Lhc Pclmlcum Exploration Promotion Project ofthe Departmcm of Mines and Geology of His Majesty’s G,ovcmmen~ d Nepal Cur lhcir a~sisuncc in prcpring rhis paper. ClECi 103 DccvlllbcrIV94 O.R. FKIEDENREICH. 0.L SLLND, U.M.S. PRADHAN and R.B. SHRESTHA 130 to 230 km. The country is naturally separated into four Group) thrust to the south (Parkash et al., 1980; Herail et al., major geographical/geological zones that parallel its long 1986). Gravity measurements and detailed field mapping indi- dimension (Frank and Fuchs, 1970; Mitchell, 1979; StRcklin, cate that the cores of at least some of these structures contain 1980; Windley, 1983). Each zone has its own characteristic pre-Siwalik rocks that are considered to be hydrocarbon explo- stratigraphy and structure and these zones are, from south to ration objectives (Friedenreich and Slind, 1989; Ebner, 1989). north, described below (Figures 2.3). The Siwalik Fold Belt and the Terai are a pat of the fore- land of the Himalayas (Parkash et al., 1980). They have 5.1 Terai some similarities with the foothills and western plains of The Terai is the Nepal portion of the Indo-Gangetic Plain Alberta and British Columbia and are the main areas of that extends from the Indian Shield in the south to the petroleum interest in Nepal. Siwalik Fold Belt to the north. The Plain is a few hundred metres above sea level and is underlain by a thick, relatively 5.3 Lesser Himal flat-lying sequence of Mid to Late Tertiary molasse (Siwalik The Lesser Himal is a wide, stratigraphically and strut- Group) which unconformably overlies subbasins of early turally complex zone that lies immediately north of the Tertiary to Proterozoic sediments (Surkhet, Gondwana and Siwalik Fold Belt and is separated from it by the south-verging Vindhyan Groups) and igneous and metamorphic rocks of main boundaty fault (MBT). The majority of the Lesser Himal the Indian Shield (Agrawal, 1977; Acharyya and Ray, 1982; is composed of thrust sheets and nappes of metasediments and Raivemun et al., 1983). The only deep well in Nepal (Shell igneous rock of the Midland Super Group. The Group is of Nepal B.V. Biratnagar 1, TD 3530 m in 1989) was drilled in little hydrocarbon exploration interest, although the oil and gas the far eastern part of the Terai. seeps of the Dailekh area occur within the Midland Group (CPIT, !973). These seeps are interpreted to have been gener- 5.2 Siwalik Fold Belt ated in sediments below the nappes (Figure 2). The Siwalik Fold Belt is from 5 to 45 km wide and rises abruptly from the Terai along the main frontal thrust (MFT). It 5.4 Higher Himal consists of a series of ridges and valleys composed of thick The Higher Himal, which contains the spectacular peaks beds of folded and fault-repeated Tertiary molasse (Siwalik of the Great Himalayan Range, Everest, Annapuma, etc., is - 80” 82” 84” 86” 30” 200km I I S-N: Approx. Location of Schematic Cross-Section Fig. 2 !6’ 0 : Biralnagar - 1 Well 82’ 84” 86’ 88” I I I I Fig. 1. Geological map of Nepal. CIEC 104 December IV94 PETROLEUM CEOLOCY “t- PvEPAL thrust southward over the Lesser Himal by the main central not adequately dated, but is intcrprcted to be older than thrust (MCT; Figures I, 2). The zone is composed of a basal Carboniferous and to range well into the Precambrian (Frank slab of metamorphic Proterozoic rocks overlain by a con- and Fuchs, 1970; StGcklin, 1980). formable sequence of Cambrian to Eocene Tethyan sedi- Duilekh Group ~ The Dailekh Group, the lower part of ments (Bordet et al., 1981). Gas seeps occur in the upper the Midland Super Group, is made up of phyllites, garnetifer- Tethyan of northern central Nepal near the village of ous schists, feldspathic greywacke, volcanics and stroma- Muktinath (Figures 1.2). tolitic dolomites (Figures 2, 7). These rocks have been raised to the low green schist facies in the centre of the Midlands 6.0 STRATICRAPHY and the metamorphic grade increases in successive thrust sheets to the north (Mitchell, 1979; Bordet et al., 1981). The The following discussion concentrates only on those rock rocks of the Dailekh Group are not considered to bc cxplo- units that have a direct bearing on hydrocarbon exploration ration objectives. (Figure 3). Lakharpata Group ~ The Lakharpata Group occupies the Vindhyun Super Group ~ Vindhyan sediments, equiva- lent to those of the great Vindhyan Basin of Northern India, upper part of the Midland Super Group and, although not are interpreted to extend beneath the Ta% and Siwalik Fold firmly dated, is interpreted to be of Late Precambrian to Late Belt. The Vindhyan is reponed to have a maximum thickness Paleozoic ages (Stiicklin, 1980: Shrestha and Shrestha, of 5250 m (Agrawal, 1977; Srivastava et al., 1983) although 1983). It is unconformably overlain by frequently organic at least 12 000 m of pre-Tertiary seismic reflections have dark shales and sandstones of the Gondwana and Surkhet been observed under the Terai. The Vindhyen, which is con- Groups. This Group and its Vindhyan equivalent are an sidered to be a major hydrocarbon exploration objective, important exploration objective. consists of an unmetamorphosed sequence of stromatolitic The Group contains the following formations and mem- limestone, shale and sandstone. Although dating is difficult, bers: the Vindhyan is considered to be equivalent to at least a part Sangram Fm: Greenish grey to black splintery shale, con- of the Lakharpata Group of the Lesser Himal.